1. Field of the Invention
The present invention relates generally to sonichemical methods of making carbide precursors and methods of making these precursors, and more specifically to such precursors that comprise polysilanes as well as methods of making these polysilanes.
2. Discussion of the Related Art
Silicon carbide (SiC) is a ceramic material that has a variety of uses. For example, SiC can be used as a high temperature resistant material, such as in boiler furnaces and steam reforming operations. In addition, SiC can be used as an abrasive material for cutting or grinding metals. In preparing SiC, polysilanes are often used as precursors, with the pyrolysis of these polysilanes resulting in the desired SiC material. It is generally believed that branching and cross-linking of a polysilane or polycarbosilane precursor is necessary to afford high ceramic residue yields of SiC when the polymer is pyrolyzed. There are various ways of achieving high molecular weight branched and/or cross-linked polysilanes and hence high ceramic yields.
One approach to preparing such polysilanes includes the reductive coupling of RSiCl.sub.3 compounds by potassium/sodium (Na/K) alloys to yield soluble polymer networks, (RSi).sub.n. This technique is disclosed in J. Am. Chem. Soc. 110, 2342 (1988) and U.S. Pat. No. 4,808,685.
The use of alkali metals for the combined reductive-dehydrogenative coupling of silanes is also disclosed in Am. Ceram. Soc. Bull. 62, 912 (1983); Polymer Preprints 25, 1 (1984); and U.S. Pat. No. 4,472,591. According to this method, MeHSiCl.sub.2 and vinyl chlorosilanes are reacted with K or Na to produce polysilanes (eqs. 1 and 2). ##STR1##
As disclosed in U.S. Pat. Nos. 4,537,942, 4,611,035 and 5,091,485; PCT International Patent Application WO 93/14,164; T. G. Wood Ph.D. Thesis, Massachusetts Institute of Technology, Chapter 4 (1984); and Chem. Abstr. 120:198710u (1993), partly crosslinked polymethylsilane has been prepared by the reaction of MeSiHCl.sub.2 and Na in THF (eq. 3). ##STR2##
An alternate approach to polysilane synthesis involves the coupling of oligomethylsilane. For example, J. L. Robison, Ph.D. Thesis, Massachusetts Institute of Technology, Chapter 2 (1992), J. Am. Ceram. Soc. 75, 1300 (1992) and U.S. Pat. No. 5,204,380 disclose the reaction of oligomethylsilane with a catalytic amount of an early transition metallocene derivative. These metallocene derivatives induce a dehydrogenative coupling reaction which yields branching and crosslinking and consequently affords a polymer whose pyrolysis gives a high ceramic yield of near-stoichiometric SiC. In addition, the use of LiAlH.sub.4 as a cross-linking catalyst for oligomethylsilane to produce a high molecular weight polymethylsilane has been disclosed in Inorganic and Organometallic Oligomers and Polymers, Proceeding of the 33rd IUPAC Symposium on Macromolecules, p. 23 J. F. Harrod and R. M. Laine, R. M., Eds.; Montreal, Canada (1991); and Appl. Organomet. Chem. 8, 95 (1994).
Cyclic oligosilanes have been synthesized by reacting chlorohydrosilanes with lithium or lithium-alkali metal alloys, as disclosed in U.S. Pat. No. 4,276,424. In this reaction Si--Si bonds are formed by the removal of halogen and hydrogen from the chlorohydro silane by action of alkali metals (eq. 4). EQU 2RR'SiHX+2M.fwdarw.(RR'Si).sub.n +2MX+H.sub.2 .uparw. (4)
In at least some prior art arrangements processing conditions or products are not ideal where transition metal catalysts are used, residual transition metal can exist in the ultimate product, which can be disadvantageous in some circumstances. In some processes, polysilane product is not as highly branched as would be optimal to result in high ceramic yield. It remains a challenge in the art to prepare SiC precursors using relatively inexpensive, readily available and safe materials such that the polysilanes can produce comparatively pure SiC in high yield. In particular, it would be advantageous to synthesize such precursors comprising polysilanes that have these desirable characteristics.